1) Field of the Disclosure
The disclosure relates to lithium niobate electro-optical modulators, and more particularly, to a lithium niobate electro-optical modulator having a doped semiconductor structure for the mitigation of DC bias drift.
2) Description of Related Art
Electro-optical modulators are optical devices in which a signal-controlled element displaying electro-optic effect is used to modulate a beam of light. Electro-optical modulators are key components for high-speed optical transmission systems. Such electro-optical modulators are typically made from lithium niobate (LiNbO3), referred to as “LN”, because of its high electro-optic coefficient and high-quality crystals. LN modulators are primarily used as electro-optical modulators that convert high-speed electrical signals to optical signals for long distance communication systems, such as free space laser communication systems for satellite and terrestrial applications, and terrestrial and underwater fiber optic communication systems. The design of LN electro-optical modulators typically employ waveguides fabricated on a planar substrate in a Mach-Zehnder configuration (see FIG. 1).
An occurrence associated with known LN electro-optical modulators is the occurrence of DC (Direct Current) bias drift caused by undesirable charge generation and charge redistribution in the device. DC bias drift is a change in the output voltage of a power supply used to bias the modulator at a certain operating point, over a certain period of time, Steady increases to the voltage required to maintain the bias condition can cause a control system reset to occur, which can result in loss of data. In addition, the drift in DC bias voltage of LN electro-optical modulators results in a phase shift of relative intensity. Over time, this DC bias voltage can no longer be corrected for or compensated due to growth in the size of the bias. DC bias drift manifests itself in a slow drift in the DC bias voltage of the modulator required to maintain a fixed output light intensity when the device is operated in the Mach-Zehnder intensity modulator configuration. Known devices exist for controlling such DC bias drift. For example, feedback loops may be used to monitor and adjust the DC supply voltage to maintain proper operation. However, such feedback loops must be frequently monitored which can be time consuming, and such feedback loops may be ineffective with satellite and space applications since the supply voltage is only in a certain range of voltage, after which it runs out. In addition, a known LN device for reducing DC bias drift is disclosed in U.S. Pat. No. 5,404,412 to Seino et al. This patent discloses an optical waveguide device with an LN substrate and a doped multi-component oxide buffer layer on top of the entire waveguide structure, that is, in both the DC and RF sections of the waveguide. The buffer layer has a lower resistivity that results in a decreased DC bias drift. However, such device has problems with reproducibility and consistency in DC bias drift mitigation using the multi-component oxide buffer layer over both the DC and RF sections. Factors that may affect the reproducibility and consistency of this known device may include, but are not limited to, the composition of the oxide compound, the various oxide deposition conditions, and the LN surface preparation prior to deposition.
Accordingly, there is a need for an LN electro-optical modulator having a doped semiconductor structure for the mitigation of DC bias drift that provides advantages over known devices and methods.